The disposal of waste materials has become a pressing issue in today's society. Environmental concerns associated with disposal of waste materials, for example, have made recycling of materials a popular alternative to landfills and incineration. Polymers constitute a large percentage of waste materials and are present among the materials in many curbside recycling programs. Co-mingled polymeric waste is often mixed with paper labels, glass, wood, etc. These contaminants must be eliminated before the materials can be processed. However, removal of contaminants also includes the separation of undesirable polymer mixtures such as polyethylene terephthalate (PET) and polyvinyl chloride (PVC). Failure to separate materials completely can lead to degradation and impurities in the recycled end products in addition to causing processing difficulties. Hand sorting and optical-mechanical scanners are often ineffective means to accomplish the necessary separations.
The effective recovery and utilization of PET from polymer waste requires that the PET be recovered free of other polymers such as polyolefins and PVC. Several methods are used to accomplish this separation.
Solvents are used to selectively dissolve individual polymers as is the case in U.S. Pat. No. 5,198,471. This approach is unattractive because of the need to use organic solvents and handle polymer solutions. Conventional techniques must be employed for extracting a polymer from a solvent in a solution. These methods create additional waste disposal and solvent handling issues.
A preferred separation technique involves aqueous flotation of the polymer waste which capitalizes on the density differences between polymers. This approach is effective in separating the polyolefins from PET because of the large density difference (e.g. 0.9 for the polyolefins versus 1.3-1.4 for the PET). However, PVC has a density very close to PET and cannot be separated from PET without some additional techniques. European Patent Nos. 469,903A2 and 512,464A1 involve the use of surfactants to assist in the float/sink phenomena.
A method of separating incompatible plastics having only slightly different densities by selective density flotation of the materials in a fluid in the vicinity of its critical point is disclosed in U.S. Pat. No. 5,126,058. A supercritical fluid can be defined as a material which is maintained at a temperature which exceeds a critical temperature and at a pressure which exceeds a critical pressure so as to place the material in a supercritical fluid state. In a supercritical state, the supercritical fluid has properties which cause it to act as both a gas and a liquid. In U.S. Pat. No. 5,126,058 a supercritical fluid is used to separate plastic components selectively in order of increasing density in succession by adjusting the temperature of the supercritical fluid in the vessel. The entire process is conducted under supercritical fluid conditions including the separation. Disadvantages with this process include high equipment costs due to the long supercritical fluid exposure time, and the separation process must be operated under high pressure conditions.
However, a need still exists for a method of separating co-mingled polymers of similar density (i.e. PET and PVC) which is more efficient and economical.